The invention concerns an electromagnetic wave modulator equipped with coupled quantal wells. The wave to be modulated may be propagated freely or it may be guided. The invention is applicable, in particular, to the modulation of infrared waves.
It is very difficult to produce modulators which, operating in the infrared region of the spectrum, function rapidly and, at the same time, possess an great depth of modulation.
Some modulators employ injection in a semi-conducting free-carrier structure. However, the transmission band of these modulators is limited by their fairly long recombination time, which may be the result of a radiative and/or non-radiative interaction.
Pockels-effect modulators are also well known. These devices use the change in the index of refraction of the semi-conducting material under the effect of an electrical field. They are, therefore, "electrooptical" modulators. Their index variation is, however, very low, resulting in the fact that, to obtain a significant effect, use must be made of devices which have a considerable length of interaction, and which are, therefore, very large. Although the electrooptical effect is quite fast, since its characteristics times are on the order of several femto-seconds, the desire to obtain a significant depth of modulation will dictate that the size of the device condition the transmission band of the modulator. Furthermore, this is especially critical in the infrared range (in relation to the visible and near-infrared ranges) because the size of the modulator must also increase as the wavelength increases.
Other well-known modulators use the principle of electroabsorption (Franz-Keldysh effect). In this case, an electric control field is applied which, when substantially raised, shifts the absorption threshold of the material (in terms of frequency). In this latter type of modulator, use must be made of semi-conducting materials possessing a forbidden-band energy very close to the energy of the band to be modulated. For example, in the case of an infrared wave having a wavelength of 10 micrometers, materials alloys II-II of Mendeleev's table are used. However, the industrial working of these materials is difficult to master and, given their slight forbidden-band energy gap, they are very sensitive to manufacturing imperfections.
For this reason, other solutions have been proposed, according to which intra-band absorption between, for example, two discrete levels of a quantal well is used. The manufacture of semi-conducting materials used to delimit these quantal wells is much more effectively achieved when III-V semi-conductors from Mendeleev's table are used.
In such a structure, the means of control of the modulation is based on a wave-pump whose amplitude is modulated and which possesses a frequency greater than that of the wave to be modulated. The wave-pump serves to occupy a discrete level of the quantal well. The absorption of the wave to be modulated thus takes place by means of a transition from the electrons (or holes) of this first discrete level of the quantal well to another discrete level. In this way, control is effected over the absorption of a wave whose frequency is equal to the difference between the energies of the two discrete levels divided by Planck's constant.
More precisely, and as shown in FIG. 1, a modulator of this kind may incorporate a structure having a quantal well (1, 2, 3) whose optical characteristics are modified by an optical pumping achieved by means of a control wave (h..nu.1) belonging to the middle infrared band. The command wave h..nu.1 is modulated by means of a conventional modulator. By modulating the amplitude of the control wave h..nu.1, it is possible to modulate the amplitude or the phase of the wave to be modulated h..nu.3.
Although they are very advantageous for certain applications, these modulators have, however, a residual disadvantage: it is impossible to optimize them so as to obtain both a large transmission band and a significant depth of modulation.
In fact, the transmission band of these modulators is limited either by the transmission band of the wave-pump, or by the recombination of the photo-created carriers at the first discrete energy level of the quantal well. In order to restrict these disadvantageous effects to that of the limitation resulting from the transmission band of the wave-pump, short life-spans of the carriers is desirable. In this case, however, for a given wave-pump power, the shorter the life-span of the carriers, the shallower the depth of modulation. It appears, therefore, ultimately that today, it has not yet been possible to obtain optical modulators which are completely satisfactory, especially when dealing with infrared light.
A first purpose of the invention consists in supplying an optical modulator in which a spatial separation of electrons and of holes in two different quantal wells has been achieved, thus making it possible to increase the life of the electron-hole pairs. This makes it, therefore, possible to reduce the power of the wave-pump required for a given intra-band absorption.
Another purpose of the invention is to make available a modulator which may be transparent, or, on the contrary, opaque in normal operation for the electromagnetic wave to be modulated.
A third purpose of the invention is the creation of a modulator which may be controlled by the electric field. Finally, since the modulator can also be controlled by the coexistence of a wave-pump and an electric field, it may be used as an AND element or in applications of the "image-recognition" type.